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US-12624429-B2 - Strong and ductile medium manganese steel and method of making

US12624429B2US 12624429 B2US12624429 B2US 12624429B2US-12624429-B2

Abstract

An ultra-strong, ductile and cheap medium manganese steel comprises in percentage by mass: 8-12 wt. % Mn, 0.2-0.4 wt. % C, 1-3 wt. % Al, 0.05-0.39 wt. % V, and the balance of Fe. The manufacturing method of the ultra-strong and ductile medium manganese steel includes the steps of: (a) hot rolling an ingot at 900-1200° C. into a steel sheet (or plate, or bar); (b) air cooling or water quenching the steel sheet to room temperature or warm rolling temperature, (c) warm rolling the steel sheet at 350-750° C. with 30-60% thickness reduction; (d) air cooling or water quenched the steel sheet to room temperature; (e) annealing the steel sheet at 600-650° C. for 0-300 minutes and (f) air cooling or water quenched the sheet to room temperature.

Inventors

  • Mingxin HUANG
  • Chengpeng Huang

Assignees

  • THE UNIVERSITY OF HONG KONG

Dates

Publication Date
20260512
Application Date
20210506

Claims (9)

  1. 1 . A method for manufacturing warm roll (WR) medium manganese steel, comprising the steps of: (a) providing ingot comprised of 8-12 wt. % Mn, 0.2-0.4 wt. % C, 1-3 wt. % Al, 0.05-0.39 wt. % V, and the balance of Fe; (b) hot rolling the ingot at 900-1200° C. to a thick steel product; (c) air cooling the steel sheet or plate, or bar to room temperature or warm rolling temperature; (d) warm rolling the steel product at 350-750° C. with 30-60% thickness reduction; and (e) air cooling the steel product to room temperature.
  2. 2 . The method of claim 1 , wherein in the hot rolling step a starting hot rolling temperature is 1200° C. and a finishing temperature is higher than 900° C., and wherein in the warm rolling step a starting warm rolling temperature is 750° C. and a finishing temperature is higher than 350° C.
  3. 3 . The method of claim 1 , wherein the ingot comprises 10 wt. % Mn, 0.2 wt. % C, 2 wt. % Al, 0.1 wt. % V, and the balance of Fe; wherein the WR medium manganese steel has an ultimate tensile strength (UTS) up to 1.6 GPa, and uniform elongation up to 33%; and wherein the WR medium manganese steel has a volume fraction of austenite before tensile test of 80% and a volume fraction of martensite is 20%, and after tensile test the volume fraction of austenite is 28% and the volume fraction of martensite is 72%.
  4. 4 . The method of claim 1 , wherein the ingot comprises 10 wt. % Mn, 0.4 wt. % C, 2 wt. % Al, 0.3 wt. % V, and the balance of Fe; wherein the WR medium manganese steel has an ultimate tensile strength (UTS) up to 1.5 GPa, and a uniform elongation up to 16%; and wherein the WR medium manganese steel has a volume fraction of austenite of 0-5% and a volume fraction of martensite of 95-100%, and after tensile test the volume fraction of austenite is 47-53% and the volume fraction of martensite is 47-53%.
  5. 5 . A method for manufacturing WR+CR+(annealing) medium manganese steel, comprising the steps of: manufacturing WR medium manganese steel according to the method of claim 4 and further including the steps of: (f) optionally annealing the steel product at 600-650° C. for 0-300 minutes; (g) optionally air cooling the steel product to room temperature; (h) cold rolling (CR) the WR steel product at room temperature with 10-35% thickness reduction; (i) optionally annealing the steel product at 200-600° C. for 0-30 minutes; and (j) optionally air cooling or water quenched the steel product to room temperature.
  6. 6 . The method of claim 5 , wherein in the hot rolling step a starting hot rolling temperature is 1200° C. and a finishing temperature is higher than 900° C.; and wherein in the warm rolling step a starting warm rolling temperature is 750° C., and a finishing temperature is higher than 350° C.
  7. 7 . The method of claim 5 , wherein the ingot comprises 10 wt. % Mn, 0.2 wt. % C, 2 wt. % Al, 0.1 wt. % V, and the balance of Fe; wherein the steps (f) (g) (i) (j) can be deleted so the annealing time is 0 min; wherein the WR+CR medium manganese steel product has a super high yield strength up to 1.8 GPa, and good uniform elongation up to 14%; and wherein the WR+CR medium manganese steel product has a volume fraction of austenite before tensile test of 40% and a volume fraction of martensite is 60%, and after tensile test the volume fraction of austenite is 27% and the volume fraction of martensite is 73%.
  8. 8 . The method of claim 5 , wherein the ingot comprises 10 wt. % Mn, 0.4 wt. % C, 2 wt. % Al, 0.3 wt. % V, and the balance of Fe; wherein the step (f) of annealing the steel product is at 620° C. for 300 minutes; wherein the step (i) of annealing the steel product is at 350-450° C. for 6 minutes; wherein the WR+CR+annealing medium manganese steel product has a super high yield strength up to 2.0 GPa, and good uniform elongation up to 20%; and wherein the WR+CR+annealing medium manganese steel product has a volume fraction of austenite before a tensile test of 50-55% and a volume fraction of martensite of 45-50% and after tensile test, the volume fraction of austenite test is 40-44% and the volume fraction of martensite is 56-60%.
  9. 9 . The method of claim 1 , wherein the thick steel product is at least one of a sheet, plate, or bar.

Description

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS This application is a U.S. National Stage Application under 35 U.S.C. § 371 of International Patent Application No. PCT/CN2021/091837, filed May 6, 2021, and claims the benefit of priority under 35 U.S.C. Section 119 (e) of U.S. Application No. 63/086,813, filed Oct. 2, 2020, all of which are incorporated by reference in their entireties. The International Application was published on Apr. 7, 2022 as International Publication No. WO 2022/068201 A1. FIELD OF THE INVENTION The present invention relates generally to strong and ductile medium manganese steel and a method of producing the same, and more particularly to a super steel that has a lower cost and is easy to manufacture. BACKGROUND OF THE INVENTION Steels play a considerably important role in the fast development of modern industries like automotive, aviation, aerospace, shipbuilding, architecture and so on. Development of advanced steels with higher strengths and better ductility are a constant goal of scientists working in the field. Such steels are expected to help in the construction of a more energy-efficiently and more environment-friendly world. A steel with high strength supports more loading with equal mass of material. In other words, with the help of high strength steel, less material is needed to satisfy the same loading condition. This important property of high strength steel makes structures in our world much lighter. For example, an automobile comprises lots of steel, which accounts for more than half of its total weight. Using a high strength steel will make the car lighter and more energy-efficient, while still proving high safety in a car crash. Besides high strength, high ductility is another important property of steel, which means it can experience a large deformation without immediate break down. A high ductile steel will also make vehicles and other structures much safer by avoiding catastrophic failure. On other hand, good ductility is also a benefit in processing and shaping of the steel to different shapes of components, for example stamping, rolling, extruding. However, improving the strength and ductility of a steel simultaneously is usually very difficult. This is known as the strength-ductility tradeoff Many researchers have dedicated themselves to the development of advanced steel with high strength and good ductility, through a variety of methods. In the automotive industry there are generally three generations of advanced high strength steels (AHSS) that have been developed in the past several decades to make cars more light-weight, energy-efficient, low-cost and safe. The first generation of AHSS includes dual phase (DP) steels, transformation induced plasticity (TRIP) steels, complex phase (CP) steels, and martensitic (MART) steels. The product of strength and elongation of these steels is around 20,000 MPa %. The second generation of AHSS includes twinning induced plasticity (TWIP) steel with a product of strength and elongation around 60,000 MPa %, but with low yield strength and high manganese content, which can be expensive. The third generation of AHSS is now being developed, with a product of strength and elongation around 40,000 MPa %, but improved yield strength and a lower amount of manganese. Medium content manganese steel, which contains 3 wt. % to 12 wt. % manganese, is an alternative way to realize the outstanding mechanical properties of the third generation AHSS. Currently some steel companies have developed types of medium manganese steel like Quench & Partitioning (Q&P) steel, which has a good balance of high strength and good ductility. The medium manganese steel is a promising steel for making super steel that breaks the strength-ductility tradeoff A few years ago, a group at the Hong Kong University developed a super steel with the chemical composition of 8-12 wt. % Mn, 0.38-0.54 wt. % C, 1.5-2.5 wt. % Al, 0.6-0.8 wt. % V and the balance of Fe preferably with the chemical composition 10 wt. % Mn, 0.47 wt. % C, 2 wt. % Al, 0.7 wt % V and the balance of Fe that shows high yield strength up to 2.2 GPa and large uniform elongation up to 16% at the same time. The details of this development can be found in PCT International Application No. WO2018035739A1. This super medium manganese steel with 0.7 wt. % V exhibits extraordinary mechanical performance but has a much lower price compared to other high strength steels like maraging steels. Despite the exceptionable mechanical properties of this super steel, it does have some limitations. First, the process of making this steel involves many steps of rolling and annealing, which are very time consuming and inconvenient for industrial manufacture. Second, the properties of the steel are very sensitive to the temperature of the last annealing process, which is not easy to control during industrial manufacture. Third, the high content of C in this super steel causes it to have a very poor welding property, which